On one end stood hundreds of supporters of President Ali Abdullah Saleh. At the other end were hundreds of anti-government protesters. In the middle, tires blazed next to shuttered shops. Plumes of smoke rose into the sky, as did the smell of burned rubber. The chants and slogans were punctuated by the sound of gunfire.

"Ya, Ali," screamed Saleh's supporters as they hurled rocks the size of baseballs.

"The people want the regime to fall," screamed the anti-government protesters as they sent rocks back toward their foes.

On Thursday, this impoverished capital witnessed the fiercest clashes since the revolts of Tunisia and Egypt unleashed a wave of pent-up resentment across the region. While relatively small in number, Yemen's anti-government activists suddenly, surprisingly retaliated with fury, fighting with metal pipes, wooden sticks, and daggers, and deepening the pressure on Saleh to find a way to calm Yemen's increasingly angry and volatile protesters.

For some observers, the chaotic scenes on Rabat Street - following weeks of peaceful protests and demonstrators running away in the face of heavily armed pro-government mobs - suggested that this strategic Middle Eastern nation had crossed a threshold, from street protests to a revolution. For others, it was a worrisome sign of what could be in store for Yemen - the country's stark poverty, tribal codes and abundance of weapons potentially ushering in the bloodiest rebellion seen in the Arab world.

"We are either approaching or have reached a tipping point where it will be very hard to reverse this popular movement," said Abdul-Ghani Al-Iryani, a political analyst. "There is a massively different feeling on the streets than a week ago."

Leaders under siege

In the face of that unrest, Saleh, a vital U.S. ally who has ruled for more than three decades, has ordered an investigation into the deaths of protesters in Aden during clashes Wednesday. That night, Saleh met with top military officers to discuss the turmoil, the state news agency Saba reported. Top officials have played down the violence, with one describing what happened on Rabat Street on Thursday as mere "hand-to-hand fighting," Saba reported.

In a telephone call with Bahrain's King Hamad bin Isa al-Khalifa, who is also facing a populist uprising, Saleh blamed "foreign agendas" for the movement to oust him and other Arab leaders, the state news agency reported. "There are schemes aimed at plunging the region into chaos and violence targeting the nation's security and the stability of its countries," Saleh told the king, according to Saba.

On Thursday, anti-government protests unfolded in seven governorates across this mountainous nation, underscoring its fragility. In the southern city of Taiz, thousands of protesters took over the main square, defiantly camping as young people did in Cairo's Tahrir Square, the focal point of the Egyptian revolution.

In the port city of Aden, thousands of protesters clashed with security forces, who used tear gas, electric stun guns and batons to disperse them. Two protesters were killed and 12 wounded, according to local news reports.

On Thursday, Sheikh Abdul-Majid al-Zindani, Yemen's most influential cleric and a strong ally of Saleh, sought to use his influence to calm the streets. Zindani, whom the United States has labeled a terrorist with links to al-Qaeda, called for a unity government, with representatives from the country's opposition in key cabinet positions, and for parliamentary elections in six months, backing a proposal by Saleh.